The present invention relates to antenna structures, including multi-band antenna structures, and techniques for the construction thereof, where an antenna is required to be mounted on a printed wiring board (PWB) or printed circuit board (PCB) that has a full ground plane (i.e. metallised layer) on a side opposed to that on which the antenna is mounted. Embodiments of the present invention also provide advantages in applications without a significant ground plane.
It is often advantageous in the design of an electrically small antenna to remove part of the ground plane on both sides of a PCB or through all the layers of a PWB as this can help to improve the bandwidth of the antenna. Unfortunately, many modern mobile telephone handsets have so many components to be fitted on the reverse side from the antenna (speakers, headphone sockets, USB connectors, display technology, etc.) that it is preferable not to remove the ground plane, either fully or partially. It is therefore desirable to find a way of designing an antenna for mounting on a PCB/PWB, the antenna having the wide bandwidth required for modern mobile telephone handsets while still retaining a full ground plane beneath the antenna.
Dielectric antennas are antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used in for example in mobile telecommunications.
The present applicant has conducted wide-ranging research in the field of dielectric antennas, and the following nomenclature will be used in the application:
High Dielectric Antenna (HDA): Any antenna making use of dielectric components either as resonators or in order to modify the response of a conductive radiator.
The class of HDAs is then subdivided into the following:
a) Dielectrically Loaded Antenna (DLA): An antenna in which a traditional, electrically conductive radiating element is encased in or located adjacent to a dielectric material (generally a solid dielectric material) that modifies the resonance characteristics of the conductive radiating element. Generally speaking, encasing a conductive radiating element in a solid dielectric material allows the use of a shorter or smaller radiating element for any given set of operating characteristics. In a DLA, there is only a trivial displacement current generated in the dielectric material, and it is the conductive element that acts as the radiator, not the dielectric material. DLAs generally have a well-defined and narrowband frequency response.b) Dielectric Resonator Antenna (DRA): An antenna in which a dielectric material (generally a solid, but could be a liquid or in some cases a gas) is provided on top of a conductive groundplane, and to which energy is fed by way of a probe feed, an aperture feed or a direct feed (e.g. a microstrip feedline). Since the first systematic study of DRAs in 1983 [LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: “The Resonant Cylindrical Dielectric Cavity Antenna”, IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R. K. and BHARTIA, P.: “Dielectric Resonator Antennas—A Review and General Design Relations for Resonant Frequency and Bandwidth”, International Journal of Microwave and Millimeter-Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247]. A summary of some more recent developments can be found in PETOSA, A., ITTIPIBOON, A., ANTAR, Y. M. M., ROSCOE, D., and CUHACI, M.: “Recent advances in Dielectric-Resonator Antenna Technology”, IEEE Antennas and Propagation Magazine, 1998, 40, (3), pp 35-48. DRAs are characterised by a deep, well-defined resonant frequency, although they tend to have broader bandwidth than DLAs. It is possible to broaden the frequency response somewhat by providing an air gap between the dielectric resonator material and the conductive groundplane. In a DRA, it is the dielectric material that acts as the primary radiator, this being due to non-trivial displacement currents generated in the dielectric by the feed.c) Broadband Dielectric Antenna (BDA): Similar to a DRA, but with little or no conductive groundplane. BDAs have a less well-defined frequency response than DRAs, and are therefore excellent for broadband applications since they operate over a wider range of frequencies. Again, in a BDA, it is the dielectric material that acts as the primary radiator, not the feed. Generally speaking, the dielectric material in a BDA can take a wide range of shapes, these not being as restricted as for a DRA. Indeed, any arbitrary dielectric shape can be made to radiate in a BDA, and this can be useful when trying to design the antenna to be conformal to its casing.d) Dielectrically Excited Antenna (DEA): A new type of antenna developed by the present applicant in which a DRA, BDA or DLA is used to excite an electrically conductive radiator. DEAs are well suited to multi-band operation, since the DRA, BDA or DLA can act as an antenna in one band and the conductive radiator can operate in a different band. DEAs are similar to DLAs in that the primary radiator is a conductive component (such as a copper dipole or patch), but unlike DLAs they have no directly connected feed mechanism. DEAs are parasitic conducting antennas that are excited by a nearby DRA, BDA or DLA having its own feed mechanism. There are advantages to this arrangement, as outlined in UK patent application no 0313890.6 of 16th Jun. 2003.
The dielectric material of a dielectric antenna can be made from several candidate materials including ceramic dielectrics, in particular low-loss ceramic dielectric materials.
For the avoidance of doubt, the expression “electrically-conductive antenna component” defines a traditional antenna component such as a patch antenna, slot antenna, monopole antenna, dipole antenna, planar inverted-L antenna (PILA), planar inverted-F antenna (PIFA) or any other antenna component that is not an HDA.
It is known from U.S. Pat. No. 5,952,972 to provide a rectangular dielectric resonator antenna having a notch at a centre of its underside. The authors clearly believe the slot is the cause of the enhanced bandwidth together with a slab of high dielectric material inserted into the slot. However, this device might be viewed in a different way as a rectangular dielectric pellet elevated by ‘legs’ at each end. It is important to appreciate that the pellet rests on a groundplane which is on the top surface of a PCB, and that the pellet is fed by a slot in the groundplane surface. There is no feed taken up to the pellet and the pellet is not described as being metallised on any of its surfaces. The antenna of U.S. Pat. No. 5,952,972 is therefore:                1. A DRA and not a BDA.        2. Not an elevated pellet clear of the groundplane.        3. Without an elevated feed.        4. Without a parasitic DEA component.        5. Not designed for inclusion in modern radiotelephone handsets.        
It is also known from IEEE Transactions on Antennas and Propagation, Vol. 43, No. 8, August 1995, pp 889-892, “Stacked annular ring dielectric resonator antenna excited by axi-symmetric coaxial probe”, Shum & Luk to provide a DRA comprising an annular ring dielectric element elevated above a groundplane and excited by a coaxial probe extending through a hole in the groundplane and into the central hole of the dielectric element. This arrangement is said to improve bandwidth. A further improvement to bandwidth is obtained by providing a second, parasitic annular ring dielectric element above the main one.